Chapter 14: Problem 20
Suppose that \(G\) is a cyclie group of order \(n\), and \(Z_{n}\) is another cyelic group of order \(n\). Show that the group of homomorphisms of \(G\) into \(Z_{n}\) is cyclic of order \(n\).
Short Answer
Expert verified
The group of homomorphisms from cyclic group G of order n to cyclic group \(Z_n\) of order n, denoted by H, is cyclic of order n and generated by \(φ_1\). With each homomorphism φ_i represented as a power of \(φ_1\): \( (φ_{1}^i)(g^k) = φ_{1}(g^{ik}) = (\omega^k)^i = \omega^{i*k} = φ_i(g^k) \), we've shown that H is cyclic with generator \(φ_{1}\) and order n.
Step by step solution
01
Understand cyclic groups
A cyclic group is a group generated by a single element. In other words, given some "generator" g in G, every element in G can be represented as a power of g. Since G and Z_n are both cyclic groups of order n, we can write:
\( G = \{g^0, g^1, g^2,..., g^{n-1}\} \) and \(Z_n = \{1, \omega, \omega^2,..., \omega^{n-1}\}\), where g and ω are generators of G and Z_n, respectively.
02
Define a homomorphism
A homomorphism is a map between two groups that preserves the group structure. Given the cyclic groups G and Z_n, we can define a homomorphism, φ, as follows: \(φ: G \rightarrow Z_n\), such that \(φ(g^k)=\omega^{ak}\), where "a" is a constant factor. The role of a homomorphism is to preserve group operations, so we have \(φ(g^k) * φ(g^l) = φ(g^{k+l})\).
03
Verify the homomorphism properties
Now we need to check the properties of the homomorphism φ:
1. Identity: The identity element of G is \(g^0\), and the identity element of Z_n is 1. We have \(φ(g^0) = \omega^{a*0} = 1\), so the identity element is preserved.
2. Inverses: For any element \(g^k\) in G, its inverse is \(g^{-k}\). The inverse of \(φ(g^k)=\omega^{ak}\) in Z_n is \(\omega^{-ak}\). We have:
\( φ(g^{-k}) = \omega^{-ak} = (\omega^{ak})^{-1} \), so the inverses are preserved.
3. Group operation preservation: For any two elements \(g^k, g^l \in G\), their product in G is \(g^{k+l}\). We have:
\(φ(g^k) * φ(g^l) = \omega^{ak} * \omega^{al} = \omega^{a(k+l)} = φ(g^{k+l})\), so the group operation is preserved.
Thus, φ is a homomorphism.
04
Prove the homomorphisms form a cyclic group
Let H be the group of all homomorphisms from G to Z_n, H = {φ_0, φ_1, ..., φ_(n-1)}. Each homomorphism φ_i is defined by a different constant factor "a" (i.e., φ_i(g^k) = \(\omega^{i*k}\)). We will show that H is cyclic by proving that it has a generator.
Consider the homomorphism \(φ_{1}\) with constant factor a = 1. We claim that \(φ_{1}\) generates H. To prove this, we need to show that every homomorphism in H can be expressed as a power of \(φ_{1}\). For each φ_i, where 0 ≤ i ≤ n-1, we have:
\( (φ_{1}^i)(g^k) = φ_{1}(g^{ik}) = (\omega^k)^i = \omega^{i*k} = φ_i(g^k) \)
This shows that each homomorphism φ_i can be represented as a power of \(φ_{1}\), so H is cyclic with generator \(φ_{1}\).
05
Determine the order of the cyclic group H
Now we need to find the order of the cyclic group H, generated by \(φ_{1}\). Note that H has n elements, since there are n different possible constant factors "a" for the homomorphisms. This means the order of H is n.
In conclusion, the group of homomorphisms from G to Z_n is a cyclic group of order n, generated by \(φ_{1}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Group Homomorphisms
In abstract algebra, a group homomorphism is a function between two groups that respects their algebraic structure. Think of it as a bridge that translates elements from one group to another without changing their essential properties.
- Preserves Structure: If you have a group G and another group H, a homomorphism \(\phi\) maps every element of G to an element in H while keeping the operation identical. This means if you apply the group operation in G, you must get the same result as first mapping the elements to H and then applying H's operation.
- Example: For cyclic groups \(G\) and \(Z_n\), a homomorphism \(\phi: G \rightarrow Z_n\) ensures that multiplying two elements in G and then mapping the result via \(\phi\) should equal mapping each element first and then multiplying them in \(Z_n\).
Identity Element
The identity element of a group is like a neutral player in the game of group operations. It doesn't change other elements when combined with them, acting as a 'zero' in addition or a 'one' in multiplication.
- Role of Identity: Every group has an identity element, denoted broadly as \(e\). For any element \(g\) in a group \(G\), \(g \cdot e = e \cdot g = g\). This consistency must hold in both groups connected by a homomorphism.
- Examples in C and \(Z_n\): In the cyclic group \(G=\{g^0, g^1, ..., g^{n-1}\}\), the identity is \(g^0\). Similarly, in \(Z_n\), the identity for the additive operation is 0 (often represented multiplicatively as 1 in defining powers).
Group Operation Preservation
For any homomorphism between two groups, the vital job is to maintain the group operation's integrity, meaning the functional structure remains intact after the mapping.
- Definition: If a homomorphism \(\phi\) maps elements from group \(G\) to \(H\), it must satisfy \(\phi(g_1 \ast g_2) = \phi(g_1) \times \phi(g_2)\) for all elements \(g_1, g_2\) in \(G\). The operation '⋆' in G corresponds to \('\times'\) in \(H\).
- Illustration with Cyclic Groups: Consider \(g^k\) and \(g^l\) in \(G\). If \(\phi(g^k) = \omega^{ak}\) and \(\phi(g^l) = \omega^{al}\), we expect \(\phi(g^{k+l}) = \omega^{a(k+l)}\).
Inverse Element
In group theory, for every element in a group, there is another element known as the inverse. When both elements are combined using the group operation, they yield the identity element.
- Inverse Functionality: For an element \(g\) in a group \(G\), the inverse is denoted \(g^{-1}\), satisfying \(g \cdot g^{-1} = g^{-1} \cdot g = e\), where \(e\) is the identity element.
- Preservation in Homomorphisms: If \(\phi\) is a homomorphism, it must respect inverses. So, for \(\phi(g) = h\), its inverse \(\phi(g^{-1})\) should satisfy \(h \times h^{-1} = e_H\), where \(\times\) denotes the operation in \(H\).
- Cyclic Group Illustration: In \(G\), the inverse of any \(g^k\) is \(g^{-k}\). For a homomorphism \(\phi(g^k) = \omega^{ak}\), the inverse is \(\omega^{-ak}\).